Texture Painting and Uv: how to control Target image?
sharing some pictures and the .blend file could help a lot
(since i don't have enough rep to comment your question i said it here)

anyway i show you the steps hope it helps:

first you have to set up a few different uv maps:




one for the texture and a few more for sides, back, top, etc.

you can use ALT+B to cut the mesh so you only select the vertices that are visible from that angle

like this


then align the uv with pictures you want to copy from 



after that go to texture paint mode and create a texture for the face

(explained in the picture) 


hope it helps